1. Field of the Invention
The present invention relates to an FM receiving apparatus for use in receiving signals in FM broadcasting or FM communications, and to an apparatus for reproducing a media which carries frequency-modulated signals recorded thereon.
2. Description of the Related Art
FIG. 6 illustrates a frequency demodulator for demodulating a signal through a conventional digital process. In the conventional digital process, an FM-IF (Frequency Modulation-Intermediate Frequency) signal is converted to a digital signal having two levels, i.e., a digitized FM-IF signal, and the digitized FM-IF signal is FM-detected by a digital frequency demodulation section such as a digital delay detector or a PLL detector.
The frequency demodulator of FIG. 6 includes a voltage comparator 301 and a digital frequency demodulation section 302.
The voltage comparator 301 compares a voltage of an input FM-IF signal with a reference voltage supplied from a voltage source (not shown) whose output voltage is fixed. The voltage comparator 301 outputs an H level digital signal when the voltage of the FM-IF signal is higher than the reference voltage, and an L level digital signal when the voltage of the FM-IF signal is lower than the reference voltage.
The digital frequency demodulation section 302 receives a digital signal from the voltage comparator 301 and outputs an encoded, frequency-demodulated signal.
In particular, in the frequency demodulator of FIG. 6, the input FM-IF signal is compared with the reference voltage by the voltage comparator 301, so as to be converted to a digital signal having two levels, i.e., a digitized FM-IF signal. The digitized FM-IF signal is then frequency-demodulated through the digital signal process by the digital frequency demodulation section 302.
Such a conventional frequency demodulator is disclosed in, for example, Japanese Laid-Open Patent Publication No. 6-188638 and Japanese Patent Application No. 9-358573.
Typically, the voltage comparator 301 of FIG. 6 comprises a differential amplification circuit having a sufficient gain and a modified circuit thereof. These circuits do not include a feedback circuit.
Since the circuits do not include a feedback circuit, there may occur saturation or cut-off of a drain current of the differential amplifier as the amplitude of the input FM-IF signal increases. In the frequency demodulator of FIG. 6, it is difficult to operate the voltage comparator 301 at a high frequency due to an influence of the charge/discharge time of the gate capacitance during a transition between the saturated and unsaturated states.
Where the input voltage is close to the reference voltage, if a noise signal is introduced into the input voltage, hysteresis noise may be output from the voltage comparator 301. When the hysteresis noise is introduced into the FM-IF signal, the frequency demodulation performed by the digital frequency demodulation section 302 may become inaccurate. This problem is more pronounced when the level of the FM-IF signal input to the voltage comparator 301 is low. The above-described problem can be avoided by providing a hysteresis characteristic to the input characteristics of the voltage comparator 301. In such a case, however, a new problem arises in that the input sensitivity is reduced.